Project Abstract/Summary (30 lines) This proposal will explore the feasibility of universal organ transplantation. This new stem cell-based approach may expand the versatility and safety of organ transplants. The eventual goal is to transplant immunologically-foreign organs into patients without recourse to long-term immune suppression. This goal will be realized by using stem-cell differentiation and immune-system replacement to reset the immune barriers that usually reject foreign organs. If successful, this might one day greatly expand the number of patients that receive life-saving organ transplants. The research proposed herein will also yield basic insight into developmental biology, stem cell biology and immunology. Organ transplants are life-saving therapies for patients with degenerative diseases or organ failure. Degenerative diseases cause ~50% of adult deaths in developed nations, and range from Parkinson's Disease to liver failure. However, the use of organ transplants to treat degenerative diseases is often impeded by the immune system, because the immune system rejects foreign tissues. Consequently, patients in need of organ transplants must wait long periods on ?the transplant list? pending the availability of scarce immune-matched organs and/or must undergo dangerous long-term immune suppression to accept mismatched organs?neither outcome is acceptable. The objective of universal organ transplantation is to transplant all human patients with tissues artificially generated from a single human embryonic stem cell (hESC) line, by overriding immune barriers that normally render this impractical. The approach is to ?reset? a patient's immune system by partially replacing it with a hESC-derived immune system and then to transplant hESC-derived tissues. In such patients, their hESC- derived immune system should permanently tolerate the transplanted hESC-derived tissue, obviating the need for lifelong immune suppression to accept the new tissue. This might one day enable many patients to receive lifesaving hESC-derived organs. To achieve this long-term goal, three capabilities must be developed: safe depletion of pre-existing human immune systems (Aim #1), generation of new human blood stem cells from ESCs (Aim #2) and generation of new human regulatory T cells from ESCs (Aim #3). As such these projects will form the three parallel lines of investigation of the current proposal to test the feasibility of universal organ transplantation, starting from basic research and assays in humanized mouse models.